The present invention relates to a controller of an area sensor having a plurality of optical axes provided within a detection area, each optical axis connecting a set of a light-projecting device and a light-receiving device. In particular, the invention relates to the safety of extension of a controller for use with a plurality of such area sensors.
An area sensor is a kind of switch that comprises a light projector having light-projecting devices and a light receiver having light-receiving devices, a set of one light-projecting device and one light-receiving device forming an optical axis or an optical channel. If any one of the optical axes is interrupted by a moving object, the area sensor turns on. Working as a switch, the area sensor ensures the safety of the operators of machine tools, punching machines, press machines, brakes, molding machines, automatically controlled machines, coiling machines, robots, casting machines and so forth. In the case of a press machine, the area sensor is positioned in a detection area which is the dangerous zone of the machine and when fingers or any other part of the operator's body enters the detection area and interrupts a particular optical axis, the sensor detects that phenomenon and takes an immediate protective action by shutting down the machine or issuing a warning signal.
The area sensor is also used in an automatic production line at plant, where it detects the presence or absence of a moving article and signals for a transfer to the next step upon detecting the article. In this case, the area sensor works as a sensor for automatic control.
An area sensor 1 of the type is shown in FIG. 9 and it comprises a light projector 2 in which a plurality of light-projecting devices 21 such as light-emitting diodes (LEDs) that emit infrared or other radiations are spaced on a specified pitch (in FIG. 9, eight light-projecting devices are provided), a light receiver 3 in which a corresponding number of light-receiving devices 31 such as photodiodes 31 that are spaced on a specified pitch in correspondence with the light-projecting devices 21 so that they receive optical axes 9 which are the infrared beams emitted from the light-projecting devices 21 in the light projector 2, and a controller 4 that controls both the light projector 2 and the light receiver 3 via cables 7. The light projector 2 and the light receiver 3 are provided in a face-to-face relationship such that the projector 2 is positioned on one side of the detection area where the operator of a press machine or the like must be protected whereas the receiver 3 is positioned on the other side of the detection area. Optical beams issued from the light-projecting devices in the light projector 2 travel to the corresponding light-receiving devices in the light receiver 3 and the interruption of any one optical beam is detected. The light receiver 3 is also equipped with an indicator 8 that signals the operating status of the area sensor.
In accordance with the control by the controller 4, the light-projecting devices 21 in the light projector 2 emit cyclically in sequence (e.g. from down to up) and with synchronism being ensured between a particular light-projecting device 21 and the corresponding light-receiving device 31 in the light receiver 3, only the corresponding light-receiving device 31 is rendered to be capable of light reception whereas the other light-receiving devices 31 are incapable of light reception. The reason for ensuring that only one corresponding light-receiving device 31 at a time is rendered to be capable of light reception is that the light from a particular light-projecting device 21 is not necessarily launched into the corresponding light-receiving devices 31 and there may be a case in which the same light is also launched into nearby light-receiving devices 31 as light of a comparatively high intensity. In other words, the conventional area sensor is of such a design that the light reception signals from all light-receiving devices 31 are collectively fed into a single binarizing circuit and, although a particular axis is interrupted by an object that has entered the detection area, the light launched into nearby light-receiving 31 causes the overall signal level to exceed a threshold and the sensor will erroneously determine that the projected light is being received by the light receiver and thus fails to achieve correct detection of the object that has entered the detection area.
On the other hand, if detection is continued with only one optical axis being cyclically rendered effective at a time, the entrance of an object or fingers or some other part of the human body into the detection area interrupts the optical axis 9 in the affected position so that it is no longer received by the corresponding light-receiving device 31, whereupon the sensor issued a warning signal or shuts down the machine to ensure safety for the operator.
FIG. 8 is a block diagram for the area sensor under consideration. The area sensor 1 comprises the light projector 2, the light receiver 3 and the (master) controller 4.
The light projector 2 comprises a desired number N of light-projecting devices 21 (211, 212, . . . 21N) in the form of light-emitting diodes or the like that are spaced on a desired pitch, say, 40 mm, N light projecting circuits 22 (221, 222, . . . 22N) for driving these light-projecting devices 21, a gate array 23 that scan controls the N light-projecting circuits 22 on a time-sharing basis to perform the necessary processing for detecting abnormalities and displaying the detected abnormality, an indicator circuit 24 for signaling the operating status of the area sensor, a clocking oscillator circuit 25 and a power supply circuit 26. In the illustrated case, the operation of the light-projecting circuits 22 is controlled by using the gate array 23. Needless to say, the gate array may be replaced by other control devices such as a CPU.
The lights receiver 3 comprises a desired number N of light-receiving devices 31 (311, 312, . . . 31N) in the form of phototransistors or the like that are spaced on the same pitch as the light-projecting devices 21 in the light projector 2, N light-receiving circuits 32 (321, 322, . . . 32N) for performing I-V (current voltage) conversion on the light reception signals from the respective light-receiving device 31, a gate array 33 that scan controls the N light-receiving circuits 32 on a time-sharing basis in synchronism with the corresponding light-projecting devices 21, an indicator circuit 34 that displays the status of the associated area sensor, a clocking oscillator circuit 35, a power supply circuit 36, a light reception signal processing circuit 37 that collectively amplifies, binarizes and detects the light reception signals from the light-receiving circuits 32, a detection signal output circuit, and an output circuit 38 for delivering sync signals. Besides synchronous scan control, the gate array 33 performs auxiliary detecting operations, abnormality detecting operations and processing for displaying the results of detection. As in the case of the gate array 23, the gate array 33 of course can be replaced by other control devices such as a CPU.
The controller 4 includes a control circuit 41 in the form of a gate array; the control circuit 41 receives an external input from an external input circuit using an input terminal, a mode setting from a mode setting circuit using a DIP switch, and a signal indicative of the status of light projection from a sensor connector 42 to the light projector via a light projection status signal input circuit; the control circuit 41 also receives the aforementioned detection output and a system sync signal from a sensor connector 43 to the light receiver via a detection signal input circuit and a sync signal input circuit, respectively. The control circuit 41 delivers the following three signals from an output circuit 47: a sync signal, a mode setting signal and a shutdown output signal that are respectively sent to the light projector 2, the light receiver 3 and the press machine or the like. In addition, the control circuit 41 causes an indicator LED to be lit for indicating via an indicator circuit 48 that the controller 4 is in operation and it receives from a key switch a signal for disengaging the controller 4 from a locked-out state. The controller 4 also includes a system power supply circuit 49 for supplying operating energy to the light projector, the light receiver and the controller, an internal power supply circuit 46 with which the voltage from the system power supply circuit 49 is converted to a constant operating voltage for the control circuit, and an oscillator circuit 45 for clocking the controller.
The above-described circuit configurations of the light projector, light receiver and the controller are just one example and other configurations may be employed; for example, the circuits in the controller 4 may optionally be transferred to the light projector 2 or the light receiver 3; conversely, the circuits in the light projector 2 or the light receiver 3 may be transferred to the controller 4. Thus, the individual circuits of interest are by no means limited to those shown in FIG. 8.
The foregoing description applies to the case where only one area sensor is employed. If more than one area need be detected, a corresponding number of such area sensors are necessary. FIG. 7 shows a conventional case of the controller circuit configuration that is employed with more than one area sensor.
In the case shown in FIG. 7, two area sensors 1 and 1' are employed. According to one approach, area sensor 1 includes a light projector 2, a light receiver 3 and a master controller 4 that have the same circuit configurations as the light projector 2, the light receiver 3 and the controller 4 that are shown in FIG. 8. Similarly, the area sensor 1' includes a light projector 2', a light receiver 3' and a master controller 4' that also have the same circuit configurations as shown in FIG. 8. Thus, the two master controllers 4 and 4' each have a detecting portion and an output circuit and as a system they are independent of each other. Such master controllers are used in parallel. A problem with the control using the master controllers 4 and 4' that are operated in a totally independent manner is that even if a light-receiving device in one area sensor which should inherently be prevented from receiving light may happen to receive the light from a light-projecting device in the light projector in a nearby area sensor. In order to avoid this dangerous situation, the light projectors 2 and 2' are synchronized and all the light-projecting devices are scan lit sequentially at specified intervals that take into account the timings of light emission from those light-projecting devices. All light-receiving devices in the light receivers 3 and 3' that pair with the light projectors 2 and 2' as allowed to perform scanned light reception in synchronism with those emission timings.
Instead of the control using the master controllers 4 and 4' in a totally independent manner, the master controller 4 may be used as a common controller of the two area sensors whereas the master controller 4' is dedicated to the transfer of signals between the two sensors. Between these two extremes, there are many approaches depending on the way how the responsibility for control is shared by the two controllers and the choice of a specific approach is a design matter.
If the power supply circuit is connected to the system power supply for the master controller via an extension connector which is indicated by 44 in FIG. 8, the problem of cumbersomeness in installation is solved since there is no need to thread power cables.
Speaking of a capability for the addition of area sensors, if 64 optical axes can be scanned within a period of 7 ms, the number of operable optical axes can be increased up to 64. Needless to say, the master controller 4 can perform the necessary control even if no optical axes area added. Assume here that the area sensor 1 has 16 optical axes; in this case, lines corresponding to 16 optical axes are turned on and those corresponding to the remaining 48 optical axes are turned off to enable the necessary control. Thus, there is no need to replace the master controller 4 by a different one in response to an increase or decrease in the number of optical axes. Not a single part of the electric circuits need be changed.
The master controller is also required to recognize the addition of a slave controller. One possible way to meet this need is by the operator selecting between settings with a switch or like means so as to ensure that the master controller can recognize how many slave controllers have been added. However, this approach of having the operator recognize the number of extensions of slave controllers has been problematic since he may inadvertently forget to enter a setting or make a wrong setting.
One possible way to recognize the addition of a slave controller is shown generally in FIGS. 6A and 6B. In FIG. 6A, a master controller 4 has an extension recognizing circuit and a slave controller 5 to be connected to the master controller 5 also has an extension recognizing circuit. The extension recognizing circuit in the master controller 4 has a control circuit 1 to which a line voltage Vcc is connected via a resistance R; the recognizing circuit also has a control circuit 2 to which a line voltage Vcc is connected via a resistance R. The extension recognizing circuit in the slave controller 5 has corresponding shorting circuits that are connected to the ground, such that when the connector on the slave controller 5 is connected to the connector on the master controller 4, the control circuits 1 and 2 are rendered to a low (L) level.
Assume here that only the master controller 4 is in operation without being connected to the slave controller 5 (see FIG. 6A). In this case, the control circuits 1 and 2 in the master controller 4 are supplied with Vcc to be rendered to a high (H) level., whereupon the extension recognizing circuit concludes that "no slave controller is added".
If the slave controller 5 is connected to the master controller 4 as shown in FIG. 6B, the shorting circuits in the slave controller 5 that are connected to the ground allows the control circuits 1 and 2 to be rendered to a "L" level, whereupon the extension recognizing circuit concludes that "a slave controller is added".
Thus, the conventional method depends upon the presence or absence of connection of the slave controller 5 to automatically determine whether the slave controller 5 has been "added" or "not added".
With the above-described approaches, there has been no problem in ordinary cases. However, if an abnormal event occurs as exemplified by the connector on the slave controller 5 coming off the master controller 4, the operator being unaware of that event may fail to reconnect the connector to the master controller despite the addition of an area sensor. In this case, an output signal for the interruption of light in a certain optical axis will not be transmitted to the associated control circuit and the press machine or some other apparatus under control fails to stop intermediately, which is a very dangerous situation.